An apparatus comprises a comparator that includes a first input, a second input and an output. The comparator is configured for measuring a difference in voltage between a source coupled to the first input and another source coupled to the second input, and providing information associated with the measured difference at the output. The apparatus also comprises a controllable current source coupled to the first input of the comparator and configured for supplying or drawing current. The apparatus also comprises a digital logic circuit that is configured for controlling an amount of current supplied or drawn by the controllable current source. The apparatus is configured for measuring a charge associated with an external source that is coupled to the first input of the comparator.